The present invention relates generally to a method of hydrolyzing lignocellulosic materials such as agricultural products and by-products, forest products and wastes and municipal solid waste to fermentable sugars by employing an improved concentrated sulfuric acid process at low temperatures and pressures. More particularly, the invention relates to a method of hydrolyzing these materials in a single step in the presence of 30 percent on greater sulfuric acid at reaction temperatures of 100.degree. C. or less or utilizing a modified single step process involving a first hydrolysis reaction utilizing a first sulfuric acid concentration and second hydrolysis reaction utilizing second, diluted sulfuric acid concentration.
An alternative to oil and natural gas is the use of biomass as a raw material in the production of valuable fuels and chemicals. Such a process requires a method of producing sugars from the carbohydrate fraction of the biomass, followed by fermentation of the resulting sugars to fuels and chemicals by employing yeast or bacteria.
Biomass is composed of three major materials: cellulose, hemicellulose and lignin in ratios of roughly 4:3:3. The cellulose and hemicellulose are carbohydrate polymers, while the lignin fraction is phenolic in nature. Biomass sources include agricultural crops, agricultural by-products, forest products and by products, municipal solid waste and other lignocellulosic materials.
To convert biomass materials to fuels and chemicals, a suitable method must be found to hydrolyze the carbohydrate fraction to sugar monomers, principally glucose and xylose. These glucose monomers can then be fermented or chemically converted to the desired end-products. The most common method used in accomplishing the hydrolysis is acid hydrolysis. In general, acid hydrolysis requires either dilute acid at high temperatures or concentrated acid at reduced temperatures. Dilute acid processes have the advantage of not requiring acid recovery but suffer from relatively low conversion efficiencies (50-60 percent). Concentrated acid processes give higher yields but require acid recovery processes to make the hydrolyses economically feasible.
Although various acids have been employed in acid hydrolysis, most processes utilize either sulfuric or hydrochloric acid. Other acids utilized include hydrofluoric, phosphoric and acetic acids. Dilute sulfuric acid processes include the Scholler process (0.5-1% H.sub.2 SO.sub.4 at 170.degree. C.) and the Madison process (0.5% H.sub.2 SO.sub.4 at 135.degree.-190.degree. C.). Many modifications to these two technologies have occurred since their introduction, particularly with the use of stagewise processes and various reactor types. More concentrated acid can be used, although a maximum concentration of only a few percent H.sub.2 SO.sub.4 is economically feasible without acid recovery.
Concentrated sulfuric acid processes include the Hokkaido process and the Nippon Mokuzai Kagaku process, both developed in Japan. The Hokkaido process utilizes three major reaction steps: a prehydrolysis of hemicellulose with steam at 180.degree.-185.degree. C. to make the wood or other raw materials more susceptible to hydrolysis, impregnation of cellulose with 80 percent sulfuric acid at room temperature, and the dilution of the solids and acid to post-hydrolyze the material at 100.degree. C. Acid recovery is by diffusion dialysis followed by neutralization of residual acid with milk of lime. The major products of the Hokkaido process are crystalline glucose, furfural, methanol, acetic acid, and gypsum.
The Nippon Mokuzai Kagakn process also utilizes a similar multi-step process in producing crystalline glucose, crystalline xylose, refined molasses and gypsum.
These and other concentrated acid processes involve several steps in hydrolyzing lignocellulose to sugars. First, a preliminary prehydrolysis step is typically used to convert hemicellulose to sugars. Acid impregnation is then used to provide good contact between the acid and the cellulose-lignin matrix. Finally, a post-hydrolysis is carried out by introducing water and heating the cellulose-lignin-H.sub.2 SO.sub.4 -water matrix.
Highly concentrated acid (typically, 80 percent or greater) is introduced during the acid impregnation step which involves physically forcing the acid into the cellulosic medium. Water is then added during post-hydrolysis in reducing the concentration to 30 percent H.sub.2 SO.sub.4 or less. Heating to a temperature of almost 100.degree. C. for 30 minutes is then required. The large difference in acid concentration steps between impregnation and post-hydrolysis makes acid recovery difficult. Also, the relatively high temperature during post-hydrolysis represents an energy cost that could potentially be eliminated.
Accordingly, there is a need for a simplified concentrated sulfuric acid hydrolysis process that eliminates high acid concentration gradients and high reaction temperatures and reaction times. An improved hydrolysis process that eliminates all but one or two reaction steps would be a vast improvement over the present state of the art.